1. Field of the Invention
The present invention relates to hydraulic systems for powering machinery, and more particularly to distributed hydraulic systems in which each hydraulic actuator is operated by a control valve assembly located relatively close to the associated actuator.
2. Description of the Related Art
With reference to FIG. 1, a backhoe 10 is a well known type of earth moving equipment that has a bucket 12 rotatably attached to the end of an arm 14 which in turn is pivotally coupled by a boom 16 to a tractor 18, thereby forming a boom assembly 15. A hydraulic boom cylinder 20 raises and lowers the boom 16 with respect to the tractor 18 and a hydraulic arm cylinder 22 pivots the arm 14 about the end of the boom. The bucket 12 is rotated at the remote end of the arm 14 by a hydraulic bucket cylinder 24.
Traditionally, the boom assembly 15 is controlled by valves located within the chassis frame of the tractor 18 and mechanically connected to levers which the operator manipulates to independently move the boom, arm and bucket. As separate valve is provided for each of the cylinders 20, 22 and 24 on the boom assembly 15. Operating one of the valves controls the flow of pressurized hydraulic fluid from a pump on the tractor to the associated cylinder and controls the return of fluid from that cylinder back to the tank on the tractor. A separate pair of hydraulic conduits runs from each cylinder along the boom assembly to the respective valve on the chassis frame. Each of these conduits is subject to fatigue as they flex with motion of the boom assembly.
More recently, there has been a trend away from mechanically operated valves to electrohydraulic valves that are operated by electrical signals. Electrical valve operation enables computerized control of the functions on the machine. In addition, hydraulic control now can be distributed throughout the machine by locating the valves for a given hydraulic function in close proximity to the hydraulic actuator, such as a cylinder, being operated by those valves. In the distributed hydraulic system, the operator in the cab of the tractor 18 manipulates joysticks or other input devices which send electrical signals to separate valve assemblies located adjacent each of the boom assembly cylinders 20, 22 and 24.
Such distributed control reduces the amount of hydraulic plumbing on the machine. In the case of the boom assembly 15, for example, only a single hydraulic fluid supply conduit and a single fluid return conduit are required to be run along that assembly in order to power the functions for pivoting the boom 16, the arm 14 and the bucket 12. In this case, the number of hydraulic conduits has been reduced to one third of those required in the traditional hydraulic control system. Reducing the number of hydraulic conduits also reduces conduit failure and the machine maintenance.
However, distributed control is not without drawbacks. In traditional hydraulic systems, the pressure produced by the pump is controlled to meet the greatest pressure demand among all the hydraulic functions being operated at a given instant in time. The pressure demands are obtained by sensing the workport pressures at the mechanical valves on the chassis frame. A mechanism selects the highest workport pressure from among all the valves and uses that pressure to control the output pressure of the pump. Either a variable displacement pump is used, or an unloader valve or similar mechanism regulates the supply conduit pressure at the outlet of a fixed displacement pump. The supply conduit pressure usually is set some amount, referred to as the “margin”, above the highest workport pressure to provide a differential pressure to meter oil from the output pressure of the pump to the workport pressure. This pump pressure control technique works satisfactorily in a hydraulic system with a centralized assembly of valves to which the actuators are connected by separate pairs of hydraulic conduits.
It has been found that a distributed hydraulic system, in which a common pair of supply and return conduits is connected to a plurality of hydraulic functions, that losses in different sections of the fluid distribution system affect operation of each of the hydraulic functions differently. For example, the loss in a hydraulic conduit section relatively near the tractor through which fluid flows to or from several hydraulic functions, affects the operation of all those functions, whereas the loss in a section through which fluid flow to or from only one hydraulic function affects operation of only that function. Furthermore, sensing the pressure at the hydraulic valves located in close proximity to the actuator being controlled does not adequately account for the conduit losses between that valve assembly and the tractor when determining the pressure level that the pump has to supply.
U.S. Pat. No. 6,718,759 describes a velocity based method for controlling a multiple function hydraulic system. That method is based on modeling each hydraulic function by an flow coefficient which represents the equivalent fluid conductance of the hydraulic branch in a selected metering mode. The equivalent conductance coefficient then is used along with the desired velocity for that function's hydraulic actuator, the metering mode and sensed pressures in the function to calculate individual valve conductance coefficients, that characterize fluid flow through each control valve of the function and thus the amount, if any, that each control valve is to open. Alternatively, this control method may be implemented using restriction coefficients, which are inversely related to the conductance coefficients, as both characterize the flow of fluid in a section or component of a hydraulic system. Conductance and restriction coefficients are generically referred to as “flow coefficients”.
This method, based on deriving flow coefficients, requires that fluid at the proper pressure be supplied to the valve assembly at each hydraulic actuator. For optimal performance, this method requires knowledge of that pressure in order to achieve the requisite amount of fluid flow and thus operate the hydraulic actuator at the desired velocity. As a consequence with this type of system, losses in different sections of the supply and return conduits of the hydraulic system become very important.